Currently, the 3rd Generation Partnership Project (3GPP) is working on radio access systems in the mobile communications system next in generation to the third-generation (3G) mobile communications system. This next-generation mobile communications system may also be referred to as “Long Term Evolution (LTE).” Alternatively, this next-generation mobile communications system may also be referred to as “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN).”
In LTE, it has been determined that control information be introduced in the MAC layer as well, so that control information is defined in the MAC layer. (See, for example, 3GPP TS 36.321.) This control information may also be referred to as “MAC Control element.”
The control information that should be transmitted frequently needs to be transmitted in the L1 layer (Layer 1). Further, the control information that may be transmitted with a longer period (at a lower frequency) may be transmitted in upper layers of the MAC layer. In the case of transmission in upper layers, radio resources are consumed. Therefore, LTE has introduced control information in the MAC layer in order to transmit control information somewhat frequently.
A description is given, with reference to FIG. 1, of the protocol stack defined in LTE. (See, for example, 3GPP TS 36.300.)
Mobile communications systems to which LTE is applied include user equipment (UE) 5. The protocol stack of the UE 5 includes the Physical layer (PHY), the MAC layer, the Radio Link Control (RLC) layer, the Packet Data Convergence Protocol (PDCP) layer, the Radio Resource Control (RRC) layer, and Non-Access Stratum (NAS).
Further, mobile communications systems to which LTE is applied include a base station (eNodeB or eNB, which stands for E-UTRAN NodeB) 6. The protocol stack of the eNB 6 includes the PHY, MAC layer, RLC layer, PDCP layer, and RRC layer.
Further, mobile communications systems to which LTE is applied may also include a mobility management entity (MME) 7. The protocol stack of the MME 7 includes NAS.
In the above-described protocol stacks, the PHY may also be referred to as “L1 (Layer 1).” Further, the MAC layer, RLC layer, and PDCP layer form the Data Link Layer, which may also be referred to as “L2 (Layer 2).”
A description is given, with reference to FIG. 2, of the data configuration of the MAC layer. A MAC Protocol Data Unit (MAC PDU) 10 includes a MAC header 10-1, zero or more MAC Service Data Units (MAC SDU) 10-2, zero or more MAC Control elements 10-3, and optionally padding 10-4.
The MAC header 10-1 includes one or more MAC PDU sub-headers 10-10, each corresponding to either the MAC SDU 10-2, the MAC Control element 10-3, or the padding 10-4. The MAC PDU sub-header 10-10 corresponding to the MAC SDU 10-2 includes multiple header fields including LCID/E/R/R/F/L.
Next, a description is given, with reference to FIG. 3, of a method of attaching a MAC Control element.
An RLC PDU 12 is divided, so that a MAC SDU 14 is configured. That is, the MAC SDU 14 is cut out from the RLC PDU 12. A MAC Control element 16 and a header 17 are added to the MAC SDU 14, so that a MAC PDU 18 is formed. The MAC PDU 18 is transmitted in a transport block size (TBS) 19 from an antenna. This transport block size may also be referred to as “Code Word (CW).” Here, a description is given of the case where the MAC SDU 14 is generated from the divided RLC PDU 12. Alternatively, multiple RLC PDUs may be united and a MAC SDU may be generated from the united RLC PDUs.
Further, in LTE, it has been determined to introduce the MIMO transmission technology that transmits different information items from multiple antennas. The introduction of this MIMO transmission technology makes it possible to further increase transmission speed. Further, it has also been determined to introduce the Multi Code Word (MCW) technology. (See, for example, Yuda, Y., K. Hiramatsu, M. Hoshino and K. HOMMA; “A Study on Link Adaptation Scheme with Multiple Code Words for Spectral Efficiency Improvement on OFDM-MIMO Systems,” IEICE TRANS. FUNDAMENTALS, VOL. E90-A, NO. 11 NOVEMBER 2007.) According to MCW, in transmitting different information items from multiple antennas, not a single information item but the multiple information items from the MAC layer are encoded in the PHY independent of each other through application of Adaptive Modulation and Coding (AMC). Further, retransmission control is also performed independently in each antenna. For example, as illustrated in FIG. 4, RLC PDUs 12a and 12b are divided, and MAC PDUs 18a and 18b to be transmitted from antennas #1 and #2 are generated from the divided RLC PDUs 12a and 12b, respectively. Each of the MAC PDUs 18a and 18b is transmitted in a corresponding transport block size TBS #1 19a or TBS #2 19b (code word CW #1 or #2) from the corresponding antenna #1 or #2.
However, the above-described background art has problems such as the following.
No method of transmitting a MAC Control element in the case of performing MIMO transmission based on the MCW technology is determined.
For example, if a MAC Control element has been transmitted from one of multiple antennas, it takes time before the MAC Control element is retransmitted in the case of occurrence of an error in the code word in which the MAC Control element has been transmitted at the receiving end.
For example, as illustrated in FIG. 5, the RLC PDUs 12a and 12b are divided, and the MAC PDUs 18a and 18b to be transmitted from the antennas #1 and #2 are generated from the divided RLC PDUs 12a and 12b (MAC SDUs 14a and 14b), respectively. Of the MAC PDUs 18a and 18b to be transmitted from the antennas #1 and #2, respectively, the MAC PDU 18a to be transmitted from the antenna #1 includes a MAC Control element 16a. The MAC PDUs 18a and 18b are transmitted from the antennas #1 and #2 in their respective transport block sizes TBS #1 19a and TBS #2 19b (CW #1 and CW #2).
If control information in which an error has occurred is frequently transmitted as the control information transmitted in the L1 layer, it is possible to transmit (retransmit) the control information immediately after the occurrence of the error because the transmission period is short. In the case of occurrence of an error in control information (L2 control information) that is transmitted somewhat frequently but with a transmission period longer than that for control information transmitted in the L1 layer and shorter than that for control information transmitted in upper layers, it is also necessary to wait until the next transmission or retransmission time. However, the effect of delay is greater because of a longer transmission or retransmission period.